Quasi-optics denotes an extension of optical techniques to microwave and millimeter-wave frequencies. A promising application of quasi-optics is in the area of millimeter-wave power generation by combining the radio-frequency (RF) power of an array of several solid-state oscillators or amplifiers. Some demonstrated quasi-optic combination techniques are explained with reference to the figures wherein like numbers refer to like parts. One such technique is "space feeding and combination of amplifiers." In this method, as illustrated in FIG. 1, the input signal is fed to an array 105 of amplifiers 111, 113, 115 through open space using radiating antenna 101 and collimating lens 103 which provide an equal phase front to the amplifier array. After the reception, amplification and reradiation of the signal by the array, the energy is recaptured by focusing lens 107 and receiving antenna 109 and routed to the load (not shown here). The amplifiers may be on a planar substrate or in a waveguide structure. Further, it may be necessary to cascade several arrays of these amplifiers to provide enough driver power to saturate the final amplifier array for maximum output power.
Another demonstrated quasi-optic combination technique is practiced in an open Fabry-Perot resonator. This technique consists of coupling the output energy from an oscillator array 203 into the fundamental beam wave of Fabry-Perot resonator 201. The resonator consists of 100% reflective mirror 205 and partially reflective mirror 207 through which energy is coupled out. In operation, energy output from oscillator array 203 is incident on partially reflective mirror 207 which reflects a portion of the output and transmits it back to the oscillator array as feed-back signal. This requires the length of the cavity to be such that the incident energy and reflected energy is in phase. As is illustrated by the double-headed arrows in FIG. 2, oscillator output and the feedback signals share the same quasi-optical paths. This combining technique relies on the unobstructed reflections from the two mirrors, with the continuously reflecting energy being pumped by the individual oscillators. However, the bias leads, matching circuits and the substrate cause loss and secondary reflections which reduce the cavity Quality factor (Q) and are liable to create undesired interference.
As can be seen from comparing the two techniques described above, oscillator architecture differs from amplifier architecture in that a part of the output signal from the active devices (oscillators, in this case) is fed back to them as input through a feedback loop and is in turn amplified. The feedback conditions for oscillation are: (1) the magnitude of the open loop insertion gain must be greater than 1 (this is achieved by a selection of the amplifiers to be used) and (2) the phase of the open loop insertion gain must be an integer times 360 degrees. When these conditions are met, the signal grows in amplitude with each cycle until the active devices saturate and the steady state oscillation is achieved.